This invention relates to the field of pressure measurement. Specifically this invention relates to an improvement in an optical technique for pressure measurement used in the field of static and dynamic foot pressure distribution measurements.
One prior art foot pressure measurement system is disclosed in R. P. Betts and T. Duckworth, "A Device for Measuring Plantar Pressures Under the Sole of the Foot," 7 Engineering in Medicine 223 (1978). As described therein, this system comprises a glass or transparent plate illuminated along two or more edges, with a thin sheet of reflective material on its upper surface, as shown in FIGS. 1 and 2. The light shining into the plate normally is internally reflected. The reflective material causes light to escape through the top and bottom surfaces of the glass plate when pressure is applied to the reflective material. The amount of light escaping is proportional to the applied pressure. Viewing the underside of the glass reveals the applied pressure distribution as a variation in light intensity. The variation of light intensity and hence pressure are conveyed to the observer by viewing the underside of the plate, either directly or using a mirror, with a monochrome television camera. In an alternative prior art embodiment, the reflected light is processed into color bands for a display on a color monitor, each band representing a specific pressure range.
A significant advance in this art was made by capturing the light intensity data in a computer system. See. e.g., C. I. Franks, et al., "Microprocessor-based Image Processing System for Dynamic Foot Pressure Studies," Medical & Biological Engineering and Computing 566 (Sept. 1983). A typical configuration for such a system is shown in FIG. 3. In this system data from the video camera is converted into digital format by a microprocessor image capture and analysis system, and the data are stored in digital memory. This system enables the observer to playback the pressure distribution data from either a static or dynamic sequence of samples and to perform further analysis of the data. With the development of this system improved calibration techniques became possible and were implemented by measuring the total vertical force applied to the top surface of the plate each time light intensity distribution was measured using force transducers. This provided the necessary data with which to calibrate the light values in terms of applied pressures.
The prior art systems depend on distinguishing background light levels from light levels caused by foot pressure by setting a threshold level, below which all light values are considered to be background light. As there is a certain amount of noise in the video data, and a degree of irregularity in the evenness of background light escaping from the glass plate, setting a threshold level can lead to loss of low levels of foot pressure data if the threshold level is set too high and erroneous data if the threshold level is set too low.